Submarine pipelines laid on the seabed are usually either embedded in trenches or laid unburied on the seabed. In the latter case, due to uneven seabed topography and surface depressions, sections of free spans may occur. These sections are subject to a variety of forces which depend on many factors, such as the length of the section, the distance between the pipeline and seabed, currents etc. Adequate support of these free spanning sections is often required in order to maintain their stability and to reduce the risk of material fatigue and damage due to stress.
Existing technologies are based on supporting the free spans by changing the seabed topography, e.g. by rock infill and/or trenching/dredging of the shoulders of the free span, by discrete supports, or a combination of these methods. This has a huge cost, especially for long spans or for spans where there is a significant distance between the bottom depression and the pipeline. Additional vessels are also required to perform the mitigating/rectification work, which is both costly and time consuming.
Reduction or elimination of the need for intervention by conventional rock infill/trenching is hence highly desirable.
NO314056 (B1) discloses a pipeline laying method which achieves a controlled thermal expansion, limited to certain expansion loops, and thus controlled back bending in the vertical plane; this also provides possibilities of adapting the shape of the pipeline to the seabed topography in order to prevent buckling over ridges and long free spans over ditches. The pipeline is bent using the straightener system on the reel ship. However, this method is only applicable to pipelines installed by reel-lay.